Lasers emitting collimated beams of coherent light have many applications in optical science and technology, including lithography, spectroscopy, communications and display technology. Due to fundamental properties of light propagation in optical resonators, most lasers emit beams having a light intensity that is extremely inhomogeneous. Specifically, the light intensity of an emitted laser beam generally follows a Gaussian distributionI(r)=(2P/πw2)e−2r2/w2,  (1)where I(r) denotes the optical power per unit area measured at a distance r from the axis of the beam, P denotes the total power of the beam, and w is the beam waist parameter, which sets the length scale over which the optical intensity declines from its maximum value to zero. The same distribution also describes, to a good approximation, the intensity profile of a beam that emerges from a single-mode optical fiber, such as is used extensively in the optical industry for conveying light.
For many applications, it is desirable that some area of interest be illuminated as uniformly as possible. For example, optical lithography, which is used to fabricate microelectronic devices, requires that the light fluence over an entire exposed region conform to tight tolerances. Laser users, therefore, frequently encounter the problem of transforming a beam having a Gaussian intensity profile to an output beam having optical intensity that is substantially uniform, e.g., a so-called flat-top profile having uniform intensity over a circular or rectangular region.
Many solutions have been proposed for transforming a Gaussian beam to a flat-top beam. Apparatuses and methods for accomplishing this are disclosed in Applicants' application Ser. No. 09/917,370 titled “System for converting optical beams to collimated flat-top beams” filed Jul. 27, 2001, which issued as U.S. Pat. No. 6,654,183 on Nov. 25, 2003, and in Applicants' U.S. Pat. No. 6,295,168 titled “Refractive optical beam that converts a laser beam to a collimated flat-top beam” issued Sep. 25, 2001, both of which are hereby incorporated by reference. These references present designs that use aspheric optical elements to convert essentially all (99.7%) of the incident optical power of a non-uniform beam into a flat-top beam having greater than 70% of the optical power with less than 5% RMS (root mean square) variation. The edges of the reshaped beam are preferably rolled off in a controlled manner, thereby allowing the beam to propagate without the intensity aberrations that would otherwise arise from diffraction effects due to hard edges. Although the low dispersion glass (silica) used in these designs produces an intensity profile in the output pupil which is essentially uniform from wavelengths of 257 nm to greater than 1.5 microns, the combined dispersion of the two elements causes the output beam wavefront to converge or diverge (i.e., become uncollimated) as the wavelength varies from the nominal design value.
Accordingly, optical designs and components for producing flat-top, achromatic output over a wide wavelength interval would be of great benefit to the optics community.